Agriculture Reference
In-Depth Information
1989). Breakdown of the antifungal dienes was delayed by
CO
2
(Prusky
et al
. 1991), hypobaric pressure (Prusky
et al
.
1984) and treatment with antioxidants (Prusky
et al
. 1995).
Cercospora spot of avocados, caused by
Pseudo-
cercospora purpurea
causes spots on fruit, which at first
form small greenish-white dots that expand into slightly
sunken irregular brown blotches. Mature lesions are rarely
larger than 0.5 cm, but the cracks and lesions formed pro-
vide entry for other fungi, particularly anthracnose
(Snowden 1990). This disease is found in Brazil
(Albuquerque 1962), South Africa (Darvas 1982),
Cameroon (Gaillard 1971), Japan (Hino & Tokeshi 1976),
Mexico (Fucikovsky & Luna 1987) and the United States
(Nagy & Shaw 1980). Up to 69% pre-harvest fruit loss on
some orchards in South Africa has been attributed to
infection with this fungus (Darvas & Kotze 1987).
Dothiorella rot, caused by
Botryosphaeria ribis
, of which
the conidial state is
Dothiorella gregaria
is found in Israel,
South Africa (Labuschagne & Rowell 1983), the United
States (Stevens & Piper 1941), and parts of South America
(Zentmyer 1961). In New Zealand this disease is caused by
Botryosphaeria parva
(Hartill
et al
. 1986) or
Botryosphaeria
dothidea
(Hartill 1991) and in Australia by
Dothiorella
aromatica
(Muirhead
et al
. 1982). This disease can invade
avocados through the body of the fruit or through the cut
stem end (Snowden 1990). Symptoms usually only appear
as fruit begin to soften after harvest, although this fungus
has been isolated from lesions on hard unripe Californian
avocados (Snowden 1990). Stem end rot, generally caused
by
Botryodiplodia theobromae
in Australia (Peterson
1978), South Africa (Darvas
et al
. 1987), the Ivory Coast
(Frossard 1964) and the United States (Stevens & Piper
1941).
Dothiorella
spp.,
Phomopsis perseae
(Peterson
1978) and
Thyronectria pseudotrichia
(Darvas
et al
. 1987)
have also been implicated in stem end rots.
Dothiorella
spp. and
Phomopsis
spp. can cause latent infection in
developing fruit (Peterson 1978). However,
Botryodiplodia
is a wound parasite, and most infections with this fungus
probably take place at harvest. In New Zealand,
Botryosphaeria dothidea, B. parva
,
Colletotrichum gloe-
osporioides, C. acutatum
and
Phompsis
spp. have all been
associated with stem end rot (Hartill 1991). This appears as
dark-brown to black discolouration which begins at the
stem and advances toward the blossom end, finally cover-
ing the entire fruit.
Dothiorella gregaria
is another cause of
stem-end rot in ripe avocados. Scab, caused by
Spaceloma
perseae
, affects young developing fruit. Raised corky
brown spots are produced on the skin which mars the
cosmetic appearance of the fruit. Wound pathogens can
gain entry to the fruit through lesions caused by this fungus
(Ramallo 1969). Scab occurs in North, Central and South
America, in the West Indies (Jenkins 1934), in New
Zealand (Hartill 1991), and in South Africa and the
Philipines (Snowden 1990). Some other fungi that can
cause post-harvest diseases of avocado worldwide, but are
rare in occurrence and are not perceived to be important
include
Alternaria
sp. in Israel (Zauberman
et al
. 1975),
Penicillium expansum
in the United States and the West
Indies (Horne 1934; Wardlaw 1934),
Fusarium
spp. in
Israel (Zauberman & Schiffmann-Nadel 1979), South
Africa (Darvas & Kotze 1987), the United States (Horne
1934) and the West Indies (Wardlaw 1934),
Pestalotiopsis
versicolor
in South Africa (Darvas & Kotze 1987),
Phytophthora citricola
which attacks fruit near the ground
in Mexico (Fucikovsky & Luna 1987) and the United
States (Koike
et al
. 1987),
Trichothecium roseum
in the
United States (Horne 1934), and
Rhizopus stolonifer
in
South Africa, the United States and Israel (Darvas & Kotze
1987; Zentmyer
et al
. 1965). In New Zealand species of
Fusarium
have also been isolated from anthracnose (Hartill
1991). In New Zealand
Colletotrichum acutatum
has been
isolated from both stem end rots and anthracnose of 'Hass'
avocados (Hartill 1991).
Colletotrichum acutatum
spores
released from infected dead twigs and fruit in the canopy,
or possibly on the orchard floor, seem to infect avocado
fruit while hanging in the tree. Most infections are probably
initiated from within the avocado tree, and the influence of
shelter belt infections does not seem to be important, and
timing of infection appears to be random throughout the
year (Everett 1994).
C. acutatum
may be a wound pathogen,
as only a few fruit became infected when unwounded fruit
were artificially inoculated throughout the season in the
orchard (Everett & Hallett 1994). Damage caused by
grading equipment was insufficient to aid infection.
Avocado blight, caused by
Sphaceloma perseae
(Myriangiales: Elsinoeaceae), found in Michoacán
(Mexico), Florida, Puerto Rico, Brazil, Africa, Peru,
Cuba, Haiti and California, attacks the fruit (in all
stages), leaves, and young branches. The affected fruit
present brown lesions of corky aspect with round or
irregular shapes at first. When these lesions grow they
can cover a large part of the fruit or the whole fruit, and
cause fissuring in leaves and branches. In the fruit, the
damages are exclusive of the exocarp, while the rest of
the fruit remain healthy. However, the lesions can be an
entry point for other organisms (Gallegos 1983). The
S. perseae
fungus requires a high relative humidity and
high temperatures for its proper development. The most
susceptible stage of the fruit is when it reaches one-third
or one-half its normal size. Damage to the fruit caused
